The present invention generally relates to a retractable assembly to be included behind a bottom hole assembly of a drillstring. More particularly, the present invention relates to a retractable underreamer having retractable arms configured to retract or engage a borehole to provide cutting while remaining positively fixed against the well bore minimizing lateral movement of the cutters during reaming.
Underreamers, in oilfield parlance, refer to downhole assemblies configured to enlarge existing boreholes. Particularly, it is often preferable to drill a reduced diameter or “pilot” hole. Underreamers function to enlarge smaller pilot holes into finished larger-diameter boreholes. Often boreholes located below the lowest string of casing require bored diameters greater than the inner diameter of the preceding string of casing. For these circumstances, a collapsible underreamer is installed behind a smaller drill bit and is run through the casing to the lower bore location. Once below the casing, the collapsible underreamer is expanded and the larger borehole is drilled. Once the larger bore is complete, the underreamer is retracted and the entire drilling assembly, bit, measurement equipment, and underreamer, is retrieved through the newly drilled borehole and casing thereabove. Prior underreamers were limited in the amount of underreaming that could be performed because of physical limitations on the size of the arms deployed. In the present invention, a 5¾ inch underreamer can be used to underream up to an 8½ inch bore, while previous underreamers would be limited to a 7½ inch bore. The new design of the arm and the lateral support offered by the key and stop block combination permits large holes with little damaging vibration from the movement of the cutters across the well bore face.